GNU bug report logs - #15326
24.3; Incorrect "variable not left unused" in destructuring cl-loop

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Christopher Wellons <wellons <at> nullprogram.com>
To: bug-gnu-emacs <at> gnu.org
Subject: 24.3; Incorrect "variable not left unused" in destructuring cl-loop
Date: Tue, 10 Sep 2013 14:25:33 -0400

The byte compiler emits an invalid warning about used "_" lexical
variables in cl-loop destructuring bindings. This was casually mentioned
in bug#8711, where destructuring-bind was fixed (presumably, since it's
fine now) but not loop.

Create a source file like so,

    ;;; -*- lexical-binding: t; -*-
    (require 'cl-lib)
    (cl-loop for (_ value) in '((a 1) (b 2))
             collect value)

Then compile,

    $ emacs -Q -batch -f batch-byte-compile tmp.el
    In toplevel form:
    tmp.el:3:1:Warning: variable `_' not left unused
    Wrote /tmp/tmp.elc

In the macro expansion, the _ is declared in a `let', then later set
with `setq', "using" it.


In GNU Emacs 24.3.1 (x86_64-pc-linux-gnu, GTK+ Version 3.8.2)
 of 2013-06-25 on sochi, modified by Debian
Windowing system distributor `The X.Org Foundation', version 11.0.11204000
System Description:	Debian GNU/Linux unstable (sid)

Configured using:
 `configure '--build' 'x86_64-linux-gnu' '--build' 'x86_64-linux-gnu'
 '--prefix=/usr' '--sharedstatedir=/var/lib' '--libexecdir=/usr/lib'
 '--localstatedir=/var/lib' '--infodir=/usr/share/info'
 '--mandir=/usr/share/man' '--with-pop=yes'
 '--enable-locallisppath=/etc/emacs24:/etc/emacs:/usr/local/share/emacs/24.3/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/24.3/site-lisp:/usr/share/emacs/site-lisp'
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Message #8 received at 15326 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> IRO.UMontreal.CA>
To: Christopher Wellons <wellons <at> nullprogram.com>
Cc: 15326 <at> debbugs.gnu.org
Subject: Re: bug#15326: 24.3;
 Incorrect "variable not left unused" in destructuring cl-loop
Date: Tue, 10 Sep 2013 17:05:52 -0400

> In the macro expansion, the _ is declared in a `let', then later set
> with `setq', "using" it.

A `setq' does not *use* a variable.  It just sets it, affecting
later uses.  If there's no later uses, the setq is just a no-op.

Fixing this will require someone to dig into the `cl-loop' macro.


        Stefan "who suspects it's not really fixed in
                destructuring-bind either"

Message #11 received at 15326 <at> debbugs.gnu.org (full text, mbox):

From: Christopher Wellons <wellons <at> nullprogram.com>
To: 15326 <at> debbugs.gnu.org
Subject: Re: bug#15326: 24.3;
 Incorrect "variable not left unused" in destructuring cl-loop
Date: Wed, 11 Sep 2013 00:47:54 -0400

Stefan Monnier <monnier <at> IRO.UMontreal.CA> writes:
> A `setq' does not *use* a variable.  It just sets it, affecting
> later uses.

I mean that the compiler counts the setq as a use even in this simple
expression:

    (let (_)
      (setq _ t))
    ;; Warning: variable `_' not left unused

> If there's no later uses, the setq is just a no-op.

Without bothering to dig into the bytecode for it, it seems that the
compiler is still emitting code for the setq:

    (defun foo ()
      (let (_)
        (setq _ t)))

    (defun bar ()
      (let (_)))

    (equal (byte-compile 'foo)
           (byte-compile 'bar))
    ;; => nil

Maybe I'm not understanding what you mean by "no-op" here.

Message #16 received at 15326-done <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: Christopher Wellons <wellons <at> nullprogram.com>
Cc: 15326-done <at> debbugs.gnu.org
Subject: Re: bug#15326: 24.3;
 Incorrect "variable not left unused" in destructuring cl-loop
Date: Fri, 27 Sep 2013 21:08:53 -0400

I have just installed a patch into trunk which changes the code
generated by `cl-loop' such that this warning should not appear
any more.


        Stefan

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